(1) Field of the Invention
The present invention relates to a semiconductor device with a Schottky junction electrode made of a compound semiconductor and its manufacturing method.
(2) Description of the Related Art
In recent years, a Field Effect Transistor (hereinafter, referred to simply as FET), made of a compound semiconductor, for example, III-V family materials such as GaAs or InP, has been widely employed for wireless communication and especially as a power amplifier, a switch and the like of a cell phone terminal. Among FETs made of GaAs out of FETs made of a compound semiconductor, a Pseudomorphic High Electron Mobility Transistor (hereinafter, referred to simply as PHEMT) is generally utilized. Here, PHEMT is a FET with good high-frequency wave characteristics including a strain channel layer generated by bonding two types of semiconductors whose lattice constants are a little different. The FETs made of GaAs, for example, include the strain channel layer generated by bonding InGaAs and AlGaAs.
In the GaAs PHEMT like this, however, an AlGaAs layer that is composed of AlGaAs has a Schottky contact with a gate electrode; parts of both sides in the portion of the AlGaAs layer that do not touch the gate electrode are exposed by recess etching. As a result, a natural oxide film is formed on the surface of the AlGaAs layer, and its surface level density increases even if the AlGaAs layer is protected by a protective insulation film. Especially when the PHEMT is a power FET, the power FET does not work well because of frequency dispersion of current characteristic.
A prior art to solve the problem is “Manufacturing Method of Field Effect Transistor” (Japanese Laid-Open Patent application No. 09-045894 (pp. 3-4, FIG. 1)). The prior art resolves the problem by using an InGaP layer that is composed of InGaP that can better restrain the formation of the natural oxide film on the surface of a semiconductor layer than AlGaAs as the semiconductor layer that has the Schottky contact with the gate electrode.
FIG. 1 is a cross-sectional diagram of a conventional GaAs PHEMT.
In the GaAs PHEMT shown in FIG. 1, an epitaxial layer 120 is formed on a semi-insulating GaAs substrate 110 that is composed of semi-insulating GaAs. Here, the epitaxial layer 120 is made up of a GaAs buffer layer 121 that is composed of a 1-μm-thick undoped GaAs material and lessens a lattice mismatch between the epitaxial layer 120 and a semi-insulating GaAs substrate 110; an AlGaAs buffer layer 122 that is composed of an undoped AlGaAs material; a channel layer 123 that is composed of a 20-nm-thick undoped In0.2Ga0.8As material and in which carriers run; a spacer layer 124 that is composed of a 5-nm-thick undoped InGaP material; a carrier supply layer 125 that is a planer-doped only one atom layer with Si, n-type impurity ions; a Schottky layer 126 that is composed of a 30-nm-thick undoped InGaP material; and an n+-type GaAs cap layer 127 that is composed of a 100-nm-thick n+-type GaAs.
Additionally, on the Schottky layer 126, a gate electrode 130 that has a Schottky contact with the Schottky layer 126 is formed; and at two parts on the n+-type GaAs cap layer 127, two ohmic electrodes 140 are formed. Further, in the vicinity of the ohmic electrodes 140, two element separation regions 150 are formed; in the vicinity of the gate electrode 130, an insulation film 160 that is composed of SiN or SiO is formed.
As is described above, the conventional GaAs PHEMT can restrain an increase in the surface level density because in the conventional GaAs PHEMT, a semiconductor layer that is composed of InGaP including In and P as constituents is used as the Schottky layer 126. Therefore, the formation of a natural oxide film on the surface of the Schottky layer is restrained.
However, the conventional GaAs PHEMT has a problem explained below.
In the process of manufacturing the conventional GaAs PHEMT, since heat of about 300° C. is added to the Schottky layer and the gate electrode, diffusion at the Schottky interface between the gate electrode and the Schottky layer occurs. As a result, a problem arises in that the Schottky characteristic deteriorates. At this time, leak current of the Schottky junction between a Gate and a Source is larger than that of the conventional PHEMT having the Schottky layer that is composed of AlGaAs, and deterioration such as strain of a device is seen also in RF characteristics.
FIG. 2 is a diagram showing forward current-voltage characteristics between a Gate and a Source of the PHEMT that has a Schottky layer that is composed of InGaP and the gate electrode that is composed of Ti. In FIG. 2, the broken line shows the forward current voltage characteristic between the Gate and the Source of PHEMT before heat processing at 400° C,, while the solid line shows the forward current voltage characteristic between the Gate and Source of PHEMT after the heat processing at 400° C.
It is apparent from FIG. 2 that the leak current at a time of low bias increases due to the 400° C. heat processing and that the Schottky junction is greatly deteriorated.